For a transistor such as a field effect transistor (FET), it is preferable that there is no, or only a minimal leak current when the transistor is off. As a transistor having excellent leak characteristics at the time when the transistor is off, a transistor using an oxide semiconductor as the channel has been receiving attention in recent years.
In the transistor using an oxide semiconductor as the channel, however, a high-resistance metal oxide layer may be formed between the oxide semiconductor and a metal electrode, and the contact resistance of the metal electrode may be thereby increased.